Stable alkaline metal solutions containing carboxymethyl dextran



United Sttes "atent iiioe STABLE ALKALINE METAL SOLUTIONS CON- TAINING CARBOXYMETHYL DEXTRAN No Drawing. Application July 30, 1956 Serial No. 600,732

9 Claims. (Cl. 252-6635) This invention relates to stable alkaline solutions of certain Water-soluble metal salts and to methods of preparing them.

This application is a continuation-in-part of application Serial No. 397,014, filed December 8, 1953, now abandoned.

More particularly, the invention is concerned with the stabilization of alkaline solutions of water-soluble salts of metals that normally yield, under alkaline conditions, metallic ions which readily form hydrous oxides that precipitate in the alkaline solution.

Water-soluble salts of tervalent chromium are typical of the metal salts to which this invention is applicable. When a water-soluble salt of tervalent chromium, such as the sulfate, is dissolved in aqueous alkaline solution, such as a solution of sodium hydroxide having a pH above about 7.5, the solution is unstable since a hydrous oxide of the, metal is formed and precipitates. The hydrous oxide is an amorphous material which is neither definitely a hydroxide nor a crystalline hydrate.

In acidic solution, tervalent chromium (Cr yields cations, usually symbolized as Cr+++ although they may be mixtures of complex green and violet forms, which have a median basicity compared to metals in general.

In alkaline solution, Cr+++ yields, either immediately, on standing at ordinary temperature, or on boiling of the solution, hydrous oxides which are similar to those of Fe+++ or Mn+++.

In alkaline solution, i. e., at pH above about 7.5, freshly peptized hydrous chromium oxide may be peptized as a sol, or even dissolved as anion to some slight extent in the same way as is hydrous aluminum hydroxide or zinc hydroxide. However, the alkaline solutions ofthe watersoluble salts of tervalent chromium are not stable but yield the hydrous oxide precipitate on standing at ordinary temperature or, more quickly, on heating. Like the hydrous oxide of other metals, chromium hydrous oxide ages on standing with progressive loss of reactivity with time. This greatly complicates the handling of chromium in alkaline solution.

The problem in handling these alkaline solutions of tervalent chromium salts and salts of metals which behave similarly to tervalent chromium salts in alkaline solution, is to prevent precipitation of the hydrous oxides. Such precipitation must be prevented both on standing of the alkaline solutions during storage and when the solutions are heated prior to use, for examplein treating textiles and leather.

In the past, numerous and diverse agents have been proposed for use in minimizing the tendency of chromium and other hydrous oxides to precipitate from the alkaline solution. Thus, various stoichiometric agents have been proposed for this purpose. These include complexing anions, chelating agents, and other agents which act by chemical combination with Cr+++ or Cr Such agents must be presen'tin the alkaline solution in amounts at least chemically equivalent to the amount of chromium v as ageing.

therein in order to efiectively sequester the chromium from precipitation.

If the stability constants of the sequestered products are low, the agent must be employed in amounts in excess of the theoretical equivalent amount. If the stability constant of the sequestered product is too high, the sequestrant has to be destroyed before the solution is used, in order to recover simpler chromium cations, and this involves additional processing of the stabilized alkaline solutions.

In either case, unless the stabilizing agent is a specific sequestrant for the particular metal to be protected from precipitation, further excesses of it must be used in practice, especially if elements which are carriers would otherwise precipitate from the solution. Specific stoichiometric agents include acids or acid salts such as salts of tartaric, gluconic, citric and ethylenediamine-tetra-acetic acid, the latter being available commercially under the trade-names Versenes and Sequestrenes.

Another type of agent which has been proposed heretofore for use in stabilizing the alkaline solutions is known as a non-stoichiometric agent. This exerts a stabilizing influence against precipitation either by thickening the solution or by stabilizing colloidal hydrous oxide par-' ticles as sols, in effect stopping the phenomenon known vegetable gums, and other inert polymers.

A disadvantage of the latter type of agents is that many of these materials are heat-sensitive, form surface films and scum on evaporation, synerize on standing and yield solutions of undesirable viscosities and capricious sensi- 1 tivity to other auxiliary agents or substances which must be used in combination or admixture with the stabilized I solutions for various purposes.

The object of this invention is to provide stable aqueous alkaline solutions of salts of chromium and the likeIand new stabilizing agents for the solutions which, while having the desirable properties of the stoichiometric and nonstoichiometric stabilizers, do'not have the disadvantages of the latter and possess advantages over them. Other objects and advantages of the invention will appear hereinafter.

In accordance with the invention, aqueous alkaline solutions of the salts of the metals of the type described are stabilized by the addition thereto of a carboxymethyldextran or alkali metal salt of carboxymethyl dextran.

The dextrans are high molecular weight bacterial polyv saccharides made up of anhydroglucose units and characterized by the fact that these units are joined by molecular structural repeating linkages of both alpha-1,6 and non-alpha-1,6 types, at least 50% of the linkages being, apparently, of the alpha-1,6 type. V

The dextrans may be obtained in various ways. For, example, the native dextrans may be obtained by enzyme synthesis from sucrose in the presence or substantial absence of bacteria and cellular debris. Thus, a sucrosebearing medium may be inoculated with a culture of a suitable microorganism, such as those of the Leuconostoc mesenteroides and L. dextranicum types, and incubated until the maximum amount of dextran is formed. This is so-called whole culture synthesis. Or the culture of the microorganism may be filtered to obtain the substantially pure enzyme and the filtrate containing it, or an aqueous solution of the enzyme precipitated from the filtrate, may be introduced into the sucrose-bearing medium and the mixture held until the dextran is produced.

The dextran obtained by either method is the native product having a high molecular weight which may be in the millions. These dextrans can be separated, by fractional precipitation, into fractions of various molecular weight, as desired. The native dextran itself, or any se- Patentedoct. 14, 1958 Specific agents of this type include glues,

& ,356.

lected fraction of given molecular weight between 2000 and that of the native dextran, may be carboxymethylated to obtain a product of the formula Na CMD where CMDJP represents. acarboxyntethyldextran ion, CMD

of average molecular weight "11 and y equals; nx, where r l ner MacamvMeGIl in e mol cular wei h f: w anhydroglucose. uni (AGU) f edex aupol mer brit l dtw t .xfi lzflOre ro s and av ugian. a e e sr eo bsti ti D. f zt yin ro about. 1.0 unto 3.0..

.. Table. I surnrnanzes. variousexamples, which are illus-.

trative. -of the inyention. Thestart ng S lutions were; ob:

tained by dissolving a water-soluble salt of tervalent.

hrom u pecifically. the sulfate. naqueeus sodium ydroxidesolution of varyinghydroxide concentratiomand curs r uss xvmethyl dextran in the. r ultin lution... The normalities. of the solutions; containing the carboxymethyl. dextran. werearrivedatt using the. un-.

equivocal; equivalent weightper. anhydroglucose unit for.

. p TABLElI Stabilized alkaline chromic solutions Unit "(LliiIDf/Gr 5.015 0H .QMD- 999 0.. 25" 0.. Color 4010, 0.041 0.14. 0:178 1'5... 18 H 1. 2. 00. 0. 041 O. 12 0. 082 2.5 H 1. 1.42 i 0. 038 1 0.18 0. 054 2.2 H 1. l. 211 1 0. 042 O. 36 0. 051 9 H- l. 1. 00 0. 038 0. 18 0. 038 2.2H V 1.

' 0.61 1.2 5.81 0.74 0'61 2.4 i Fllm High As will be apparent, the equivalent concentrations of stabilized alkaline chromium solutions are given in the table; Controlsolutions of the same pI- I, but not con taining CMD- anion, yielded precipitates in a matter of minutes inthe cold, and in a few seconds on heating.

The solutions of Table I were indefinitely stable in the coldwhen they containedvarying amounts of the carboxymethyl dextran. anion of average molecular weight about'1'00,000.

Under the heading99 C., the number. of hours, H, or minutespMin, the/solutions were boiled after a period of standing, .are shown. Under the heading 25 C. the number of hours the boiled solutions stood without precipitation occurring,'are shown.

Itis apparent, also, fromthefirst'columnof Table I, that the solutions were stabilized by varying amounts of CMDTThe colorand color intensities of thestabilized solutionswere, asshown, independentof the SMD/Cr concentrationratios, and there was no evidence that the condition of coordination of, the chromium therein was in any way different from that in thelabile control solutions, whichwere prepared simultaneously, both the stabilized and 'llnstabilized solutions being of the same'unit-intensity ofrdeeptgreen huefor identical depths, at thesame Cr concentrationsi The .l .carbcxymethyl; dextran: behaved as. a so-calledtional groups capable of comb ning with the metal ion being protected against precipitation.

That the c arboxymethyl dextrans behaved to. some extent likethe non-stoichiometric stabilizing age11 t s;(e. g.

by thickening or sol-forming action) was evidenced by the fact that the alkaline chromium solutions were stabilized effectively at CMD/C3r equivalents ratios ofless than 1.00. However, that the carboxymethyl dextrans behave predominantly as hybrids appears from thetfact that ratios of about 0.1 equivalent of CMD-/Cr effectively prevented precipitation of the hydrous oxide from the alkaline solution.

In general, a ratio of 0.09 CMD-/Cr or below is not lly. ef ct v n p ev n ng. p c pi ati nand at tho ratios the protection tends; to be p.otty,. withsomelprer cipitation occurring on unstirred evaporation of;th.e solu: tion. Above aCMD? concentratiomof a fcwwwei'ght percent, sayy5.0%, theylscosityof the-solution increases nu:

desirably, and jelly-like plaques form on boilingor ,fiWiPOz.

at-le t; di r but on f 1 t CMDm ec n non-homogen ous In general, the. preferred; QM'Dr/Cr r equivalents ratios lieb w en 0.1.and 1 i i 1 r The values given in the cclumnheaded Cr in Table I were obtained by successive. boilingdown of... a. single stacksolutip-n to remove. aliquot portions oft the aqueouslal: kaline. medium, thereby successively. and progressively inrather than of the Cr f eas ng the Cr concentration. -Asliscapparentfromthe table; concentrated iSQlll iOIlS of Gr i in. strongly. alkaline 1 solu ion maybe stabilize according tothe present method.

Suehsolutions may beextremely; viscous=and form jellies zisllysl o. masses on i coolingor on further. evaporation,

which jellies can be redissolved in water, even attenbakiug thereof at- 0., .to. restore them tor stablet alkaline tervalentchromiumsolutions.

The stabilized solutions ofcTableJ were titratedwwith hydrochloric acid, .with theresults shown invTable II.

TABLE II Ac fi t o Qffl ifl c wmiamse atie r onto or 1011-. oMDr or 1301100. .MlcAeldAdded 0.00 0.012 0.34 No.00. 1571.25 (0.011001). 0.0017. 0.070 0.31 0.10 .1s- 25.

. aroused. As shown in TableI L underthe In, all instances, precipitation of the hydrous oxide; 00-, curred when the excess sodium hydroxide present was n ut al e For m erate ss ceutrationssffl the ce s. so um yd oxi e equire .red sql e fr hly: precipitated hydrous chromium oxide is about 50%. (or 50 mo e pe ent on a mole basis). flouver m pre e yd ous chr m u ridetromiarpearin perma en ly in solutions to. which an acid or neutral chromium salt is being added, only about halif of the equivalent amount of Cr+++ may beadded safely; For dilute chromium solutions, where hydrolysis of anions and depeptization otsols isfavored, greater ratios of-hydrox yl tochromic ions heading Ml! acid added, for aunit amount of free sodium hydroxide, precipitation was: first observable whom a definite: amount: 1

.range of Cr concentrations.

of acid had been added, and flocculation became apparent on adding a small, but definite, further amount of acid.

The CMD-/OH-, CMD /Cr +/OH- ratios did not influence these results.

It will be apparent, from Tables I and II, that CMD, Whether derived from sodium carboxymethyl dextran by dissolution, or from the free ether by dissolution of the salt and subsequent neutralization, is very elfective in stabilizing Cr in alkaline solutions over an extremely wide The CMD- ion is nonpoisonous, NaCMD solutions do not synerize, and the protected, stabilized solution readily yields hydrous chromic oxide on acidification. Strong acidification results in regeneration of Cr+++, but care should be exercised in carrying out the acidification required to accomplish this, since the solubility of chromium carboxymethyl dextranate varies with and is influenced by the pH of the solution. However, as shown in Table II, it is easy to remove Cr as the hydroxide once it is desired to do so, unlike the instances in which stoichiometric agents are used, where such removal may be dilficult.

Sodium carboxymethyl dextrans of different degrees of substitution were added to solutions of chromium sulfate in aqueous sodium hydroxide and the stabilizing effect was observed. The results are given in Table III which also gives the results obtained when dextran per se was used. The pH of the solutions, calculated from the hydroxyl (OH-) concentration, was about 18.0.

anhydroglucose unit, as such or in aqueous solution, with a solution obtained by dissolving the water-soluble salt of tervalent chromium or similar metal in water containing a sufilcient amount of an alkali metal hydroxide to have a pH of at least 7.5 and up to 28.0 or even higher. The CMD-/A+ (A being the metallic cation) equivalents ratio in the solution should be from 0.1 to 5.0.

The carboxymethyl dextran or alkali metal salt thereof may be incorporated in the alkaline solution of the metal salt at any time before the unstable condition of the latter develops. If precipitation of the hydrous oxide of the metal is immediate, the carboxymethyl ether or alkali metal salt thereof should be added as soon as the solution is made up. If the precipitate appears only during stor age of the solution, or on boiling thereof, the carboxymethyl dextran or alkali metal salt thereof may be added later, after the solution has been made up but before the precipitate begins to form. Generally, it is convenient to add the CMD or NaCMD as a step in the preparation of the solution of the salt of the metal in aqueous alkali metal hydroxide.

The anion, CMD", combines with the Cr cation, to form a compound or complex which may be isolated from solution and, after drying, occurs in the form of a powder. For instance, the chromium carboxymethyl dextran compound or complex is obtained as a dry powder by adding the carboxymethyl dextran to the alkaline solution of water-soluble chromium salt, pre- TABLE III Stabilized solutions Agent 11. 'ns' Percent OH Or" 20 0 90 0 NB.2.72 BCMD 70, 000 140, 000 0. 45 0. N am-MCMD 70, 00 140, 000 0. 45 0. 25 N&2.1-2.BCMD 70, 000 140, 000 0. 40 0. 24 Naz.7-2.sCMD 0. 35 0. 23 Dextran 13-512 0 0. O. 25 Dextran B5l 0. 45 0. 25 Dextran B5 12 0. 45 0. 25 Dextran B512 0.35 0.25 Dextrnn 512 1. 0 0.2 Dextran 523 1.0 0.2 Dextran 1257 1. 0 0. 2

The values shown in the table under the heading 20 C. were obtained by allowing the solutions to stand at that temperature. Clear indicates that no precipitate appeared on indefinite storage of the solutions, whereas ppt., indicates that a precipitate appeared immediately after the solutions were prepared. When the word clear appears under the heading 90 C., it indicates that the solution could be boiled down considerably and then stored for an indefinite period of time without the appearance of a precipitate.

The addition of sodium or other alkali metal salt of carboxymethyl dextran to the normally labile solutions obtained by dissolving the water-soluble salts of tervalent chromium in aqueous alkali metal hydroxide results in stabilization of the solution in the same manner as does the incorporation of free carboxymethyl dextran.

In comparing the results of adding sodium salt of the ether -(Na CMD) to the solution with those obtained when the free ether is added, it should be noted that while the carboxylate functionality of the former has been increased by a ratio of 2.7-2.8, the hydroxyl functionality thereof has been decreased by a ratio of 0.15 to 0.10; From this, it appears that however the CMD anion acts to stabilize the alkali metal hydroxide solutions of the tervalent chromium metal salts, the stabilizing action is not due primarily to the chemical efiects of hydroxyl groups present in the stabilizing agent.

The stable solutions of the invention are obtained by simply mixing the alkali metal salt of carboxymethyl dextran, or the free carboxymethyl dextran, containing an average of about 1.0 to 3.0 carboxymethyl groups per cipitating the resulting product or complex by the addition of alcohol, and subjecting the precipitate to vacuum drying.

The dextran carboxymethylated to obtain the sequestrant or stabilizing agent of the invention may have a molecular weight between 2000 and that of native unhydrolyzed dextran, as determined by light scattering measurements. It may be very high molecular weight or native dextran, or it may be a fraction thereof having any desired molecular weight in the range stated. The fractions may be obtained by direct separation from the native product by fractional precipitation or fractional solution, or native dextran may be hydrolyzed under neutral or acid conditions, or enzymatically, and the fractions separated from the hydrolyzate, as by fractional precipitation using successive additions of a precipitant such as a water-miscible aliphatic alcohol or ketone.

The carboxymethyl derivatives may be obtained in any suitable way as for instance by the method described in the application of L. I. Novak et al., Serial No. 346,016, filed March 31, 1953, and now abandoned, according to which, briefly, the selected dextran is treated in aqueousv solution or suspension with an excess of strong alkali metal chloracetate such as sodium or potassiumchloracetate, in the presence of an excess of a strong alkali metal hydroxide such as sodium of potassium hydroxide at a temperature from 50 C. to C. for a time ranging from about 10 'minutes to about two hours. Preferably, the molar ratio of the chloracetate to dextran is between 2:1 and 12:1, the molar ratio of the.

, acetone .or" a water-miscible. alcohol.

- the transition and post-transition metals of hydto tide .tohdextran molarrratioaofithe water to, dext'ran betweenflTO-z 1. Land 120:1. -:Tl1is. results. in the. production-of rthcpcorrcspondingxalkali metal salttof the ether-which meyabe used:

asesuch tin-the :presentprocess. Or the tree-ether may be recovered from the salt by 'm-ixing the latter with water, acidiiying the mixture toa' pH'of abouttfld), (and precipitating l the c'arb'oxymethyl. den-ran b l -means of carboxymethyldextran havingw-a D. LS. of from about 1 .0

up to 3y.0 carboxymethylgroupsperAGU can be obtained.

lhe talkali rneta'l salt of thewearlioxymethylaether scorrespondingtolthe alkali netala hydro ride usedas catalyst in -th reaction is obtained-as initial product. Either the alltali metalQ-saltior -the freeether may be used in practicing this invention.

'lfheninvention has in connection with the l stabilization of aqueous alkali metal hydroxide-solutions of the waterrsoluble: chromium salts- However, the invention is of wider application.

We had that aqueous alkali metal hydroxide solutions ofthe water-soluble salts of othenmetals which present the same problem of instability .as the chromium salts and iorrnfhydrous oxide precipitates inthe alkaline medium, can be efiectively stabilized against precipitation oxides by [the-inclusion therein of carof the hydrous boxymethyl dextran or an alkali metal salt lthereof.

. Examples of the metallic elements which may be sta:

bilized in accordance with this invention include beryllium, aluminum and its congeners, scandiurn and its Bythis procedur been discussed-in detail as practiced isxbetween 5:1 ,and 115:! 1.; t ndgthe imit th invent on; excep a dean.

depa t n time he sp a d empeto be understood, therefore, that it claims. v

"til ha i aclein ed i @1- A :s able aqu ous alka ine isolation .Q ifl Ma er- 0 h gh, in: lka sa an. yields ydmus oxide tion. sa deolubetween 11.14 .coa ain ngi substance r ichmrer nts .e hyd ous: ma whie uis re lected consisting Qt zearhem ethyl e hers :Q

soluble :saltiofiarn etal,

al i ions tha n m l y readily ar w h p cip tes rom the; aline ime 1 havi gra' hydm yl Lien concentrah p e ipitation of de tra av n a molecala -weie tbetween-lam and f th d t n atire "pre en the .solution be n congeners including the lanthanides and actinides and riods. The alkaline solutions of thewater-soluble salts of these metals are prepared as described previously, by

dissolving the salt in an aqueous solution of alkalip etal '1 the lo b that t of n t e. unhy mly rmc ob elo ica ly tpl' il fifid dextre-n. sa det e s come: as nza e ageoffrn 3,0 .Ghk'bQKYFB-QYL2 18 1 .P?

D to s p the der a a d. a kali meta eal. of said; er utheam un .nhydm lu ose a baxymethsi a o m talli ca on .aqu he IQtipBI'i JL EQm 5 05 p aqueous alkali e rela ion of -a w e me al which. in a ka n lution. yi lds wh h precipitates Lf QIF! the alkali e solu io aid -sQlu- I tion having a hydra yl qntcancentrati n be w en .1

N and 5.8 N and containing a substance which prevents precipitation of the-hydrous oxideand which is a carboxymethyl other ofi ciegttran-having a molecular weight between 2099 and t that pf nat ve, hnhydrolyzed, m c

b lasi produceddW aa. a d et eiai e an i average of 1.0 to 3,.0tcarboxymethyl groups per anhydroe hydroxide of such COIICBIIU'RUOH, that the solution s,

- pH of 8.0 to 28 or even higher. p

They metal-carboxymethyl dextrans formed may he f isolated from the aqueous solut1ons as described above in connection with the chromium-carboxymethyl dextran compounds.

The stabilized solutions of the invention areuseiul fora variety of purposes, for instance in analytical, separative and tanning processes, and in any method or-for Y any purpose for which stable, alkaline solutions of the metal salts are required ordesired.

Ithas been proposed, in the past, as "for example in theUJSLpate'nt as Meurant 694,658, to include'smail amounts of sugar, molasses and so on to acid or neutral sllut ions of :rnetal salts used in electroplating. The

purposeof adding the colloids tothose solutions, and thj' function thereof, is discussed in Electrop lating, by A. H. Sanders, published in 1 950 by lnternational lfextbook Cohscranton; Pa. At page '20, the author, discussing electroplating, states:

"Addition agents.--Further assistance canb e obtained by the additioii of small quantities of colloid substances lilce gelatin, glue, albumin/etc which are so-called "additionagents. No one-knows exactly how they act-to improve th e throwing power but the fact remainsthatthey do, and they also improve the character of the deposit.

Thegelatin, sugars, molasses etc. assist in electroplating with acid andneutral solutions of various metal-salts by facilitating precipitation of the metal on the'base being plate'd l This may be compared with the results shown in Table III when dextran per so was added to the present solutions.

It willbe apparent that applicants invention involves a (different concept and discovery since it prevents precipitation, ,in alkaline solution, of the hydrous oxides of those metals which tendto yield the precipitates under alkaline conditions. Instead of facilitatingprecipitation of the metal, the carboxyn ethyldextran acts to sequester the metal from precipitation.

- glueoseunit of the dextran, gniethyl dextran present in the solution being such that the ca xyme hy r an n-me all cation eq alentsa i the amount of the carboxyin the solution is from 0,5 to5.0;

' .3. Astable aqueous alkaline solu Ion of. a water-soluble salt of a metal which, inalkaline solution, yields metallic ions that normally readily form a hydrous oxide which precipitates from the alkaline solution, said solution having a hydroxyl ion concentration between 0.14 N and 5.8 N and containing a substance which prevents precipitation pf the'hydrous oxideand which is a sodium salt of ahcarboxyrnethyl ether of dextran havingamolecular weight between 2000and. that of native, unhydrolyzed, microbiologically produced dextran, said other containing antaverage oflflito 3.0;carboxymethyl groupsper anhydroglucose unitof the -dextran, the amountof thesodium.

carboxymethyl dextran present in the solution ,beinggsuch that the carboxy-methyl anion-metallic cation .cquivalents ratio in. the: solution is from 0.5 to 5,0.

4. A stable, aqueous alkaline solution of a ,watersoluble salt of tervalent :chromium Wh qhaiin lkalin solution, yields metallic ions that normally readily-form a hydroust o xide which precipita es from the alkaline solution, saidsolution having a jhydroxyl ion concentrationabetween 0. 14N. and r5 .8 N and containing asubsiflnce which reventsiprecipitationof the hydrous oxid an which is a carboxymethyl ether of dextran having a molecular wei ht between .2000and that of native, unydrolyze rmiie o iological y pro uc d xt an, said ether containing an. averages f 1.0 to 3.0 carh aymcthy r p P e hydrogluc s uni th m unt. of the ca n/m hy de tran P sent'in h solu io e ngsueh that the carboxym t yl nionzme alli a ion equ lent ratio in .theflsolution is from 0.5 to 5.0.

5. A stable, aq eous alk l ne ol t on of a l.fate of tervalent chromium which, in alkaline solution, yields metallic ions that normally readily form a hydrous oxide.

which precipitates from the alkaline solution, said solution having a hydroxyl ion concentration between 014 Nand 5.8 N and containing a substance which prevents precipitation of the hydrous oxide and which is a carataa mallr.re dilrionna hydrous oxid boxymethyl ether of dextran having a molecular weight between 2000 and that of native, unhydrolyzed, microbiologically produced dextran, said ether containing an average of 1.0 to 3.0 carboxymethyl groups per anhydroglucose unit, the amount of the carboxymethyl dextran present in the solution being such that the carboxymethyl anion-metallic cation equivalents ratio in the solution is from 0.5 to 5.0; I

6. In the preparation of an aqueous alkaline solution of a water-soluble salt of a metal which, in alkaline solution, yields metallic ions that normally readily form a hydrous oxide which precipitates from the alkaline solution, the improvement which comprises preventingvprecipitation of the hydrous oxide by incorporating in an aqueous alkaline solution of the metal salt having a hydroxyl ion concentration between 0.14 N and 5.8 N, an amount of a substance selected from the group consisting of carboxyrnethyl ethers of dextran having a molecular Weight between 2000 and that of native, unhydrolyzed, microbiologically produced dextran which ethers contain an average of L to 3.0 carboxymethyl groups per anhydroglucose unit of the dextran, and alkali metal salts of the ethers such that the carboxymethyl anion-metallic cation equivalents ratio in the solution is from 0.5 to 5.0.

7. In. the preparation of an aqueous alkaline solution of a water-soluble salt of a metal which, in alkaline solution, yields metallic ions that normally readily form a hydrous oxide which precipitates from the alkaline solution, the improvement which comprises preventing precipitation of the hydrous oxide by incorporating in an aqueous alkaline solution of the metal salt having a hydroxyl ion concentration between 0.14 N and 5.8 N, an amount of a carboxymethyl ether of dextran having a molecular weight between 2000 and that of native, unhydrolyzed, microbiologically produced dextran, which ether contains an average of 1.0 to 3.0 carboxymethyl groups per anhydroglucose unit of the dextran such that the carboxymethyl anion-metallic cation equivalents ratio in the solution is from 0.5 to 5.0. I

8. In the preparation of an aqueous alkaline solution of a Water-soluble salt of tervalent chromium which, in alkaline solution, yields metallic ions that normally readily form a hydrous oxide which precipitates from the alkaline solution, the improvement which comprises preventing precipitation of the hydrous oxide by incorporating in an aqueous alkaline solution of the metal salt having a hydroxyl ion concentration between 0.14 N and 5.8 N, an amount of a carboxymethyl ether of dextran having a molecular weight between 2000 and that of native, unhydrolyzed, microbiologically produced dextran, which ether contains an average of 1.0 to 3.0 carboxymethyl groups per anhydroglucose unit of the dextran such that the 'carboxymethyl anion-metallic cation equivalents ratio in the solution is from 0.5 to 5.0.

9. In the preparation of an aqueous alkaline solution of a sulfate of tervalent chromium which, in alkaline solution, yields metallic ions that normally readily form a hydrous oxide which precipitates from the alkaline solution, the improvement which comprises preventing precipitation of the hydrous oxide by incorporating in an aqueous alkaline solution of the metal salt having a hydroxyl ion concentration between 0.14 N and 5.8 N, an amount of a carboxymethyl ether of dextran having a molecular weight between 2000 and that of native, unhydrolyzed, microbiologically produced dextran, which ether contains an average of -1.0 to 3.0 carboxymethyl groups per anhydroglucose unit of the dextran, such that the carboxymethyl anion-metallic cation equivalents ratio in the solution is from 0.5 to 5.0.

References Cited in the filelof this patent UNITED STATES PATENTS 694,658 Meurant Mar. 4, 1902 

1. A STABLE AQUEOUS ALKALINE SOLUTION OF A WATERSOLUBLE SALT OF A METAL WHICH, IN ALKALINE SOLUTION, YIELDS METALLIC IONS THAT NORMALLY READILY FORM A HYDROUS OXIDE WHICH PRECIPITATES FROM THE ALKALINE SOLUTION, SAID SOLUTION HAVING A HYDROXYL ION CONCENTRATION BETWEEN 0.14 N AND 5.8 N AND CONTAINING A SUBSTANCE WHICH PREVENTS PRECIPITATION OF THE HYDROUS OXIDE AND WHICH IS SELECTED FROM THE GROUP CONSISTING OF CARBOXYMETHYL ETHERS OF DEXTRAN HAVING A MOLECULAR WEIGHT BETWEEN 2000 AND THAT OF NATIVE, UNHYDROLYZED, MICROBIOLOGICALLY PRODUCED DEXTRAN, SAID ETHERS CONTAINING AN AVERAGE OF FROM 1.0 TO 3.0 CARBOXYMETHYL GROUPS PER ANHYDROGLUCOSE UNIT OF THE DEXTRAN, AND ALKALI METAL SALTS OF SAID ETHERS, THE AMOUNT OF THE DEXTRAN DERIVATIVE PRESENT IN THE SOLUTION BEING SUCH THAT THE CARBOXYMETHYL ANION-METALLIC CATION EQUIVALENTS RATIO IN THE SOLUTION IS FROM 0.5 TO 5.0 